WEBVTT
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(plucked string arrangement)
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Since about three decades,
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satellites monitor routinely the ocean.
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We have hydrometers on both satellites that measure
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the evolution of sea surface temperature.
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We have also satellites that measure sea surface salinity,
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for example a small satellite launched
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some years ago by the European Space Agency.
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Satellite altimetry, it's a major technique now
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that measures the sea surface topography
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from which we derive ocean currents
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and sea level rise, of course.
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We have multispectral imageries that measure ocean colour.
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Quantities that give us information about
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phytoplankton concentration and information
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on marine ecosystems, and space gravimetry,
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a technique that identically measures
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the changing mass of the ocean.
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Used for example to learn the ice melt
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or directions of weather on which continents.
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Satellite altimetry is a whole technique.
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So first satellite well launched in the mid '70's,
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however, we had to wait until the beginning of the '90's
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with the launch of the top exposé done mission
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developed jointly by NASA in the United States
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and CNES in France.
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And this date really marks beginning
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of high-precision altimetry.
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And the beginning of oceanography from space.
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So this mission followed by many of us,
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Jason-1 Jason-2, NISAR from ISRO
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Cryosat also from ISRO.
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All these mission are now routinely used
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to derive sea-level rise.
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A very important parameter with the new generation
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of altimeter satellites.
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We are able to measure the sea surface height
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with the position of one to two centimetres.
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Which is absolutely incredible.
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Compared to the previous mission for which
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the uncertainty to a sea surface
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height measurement was in the order
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of several decimeters.
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High precision satellites altimetry,
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show us that the sea level is rising.
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At a rate of about 3.3 millimetres per year.
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In terms of globally.
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We have clear evidence that the global
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main sea level is rising.
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And compared to what we know from the previous decades,
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based on high gauge observations.
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We observed that there is an acceleration
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of about a factor of two.
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Since the beginning of the '90's
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compared to the previous decades.
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So because we have at our disposal
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a number of observing systems,
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we can also quantify the cause of this
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global mean sea-level rise.
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So there are two main causes.
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One is ocean warming.
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Because the ocean is warming, the sea water is expanding.
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So the sea level is rising, so this is one cause.
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And the second contribution is land-ice melt.
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And that is an important observation
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made by altimeter satellites is that
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sea level is not rising uniformly.
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And in some regions for example
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the Western Pacific, the Indian Ocean,
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north of Australia,
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sea level has been rising at a rate
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three to four times larger or faster
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than the global mean rise.
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For example in the Philippine region,
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since the beginning of the '90's
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the sea level has been rising by more than 25 centimetres.
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Which is a huge number.
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And we know now that non-uniform
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thermal expansion is the main cause
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of this regional viability in the rate of sea-level rise.
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How much the ocean will change
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in the coming decades will really depend
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on the future of greenhouse gases emission.
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If we continue to emit carbon dioxide
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and methane at the same rate as today,
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for example the global mean sea level
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will be higher than it is today
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by let's say around 75 centimetres.
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By 2100.
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And it is already the case now,
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there will be important viability,
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regional viability around this number.
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In particular it is expected that the Arctic
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will be even more elevated as well as the tropics.
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We expect an amplification by about 30%.
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Above the global mean rise.
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If we decide at the cup 21
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to reduce greenhouse gases emission,
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following the two degree sea warming target,
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in that case even with zero emission
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at the end of this century, sea level
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will be rising and we in that case
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what climate models simulate is an elevation
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compared to today of about 40 centimetres.
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But we see that even in that case,
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sea level will rise.
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And this is because of two factors,
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one is the contamination of the ocean.
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That have already committed to about 33%
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of the heat excess stored in the climate system
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since a few decades because of human activities,
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because of greenhouse gases emission.
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And the second factor is the long lifetime
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of carbon dioxide.
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These numbers are the position
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of climate models, but beside the climate projection
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we clearly need to continue the observation.
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To see how all the climate system is evolving.
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And if the projection, prediction,
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are in line with the observation.
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Which is absolutely fundamental
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that we have a system observing system
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in particular from space.
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If we do nothing in terms of greenhouse gases emission,
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as far as the ocean is concerned what we are expecting
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is a increase of ocean temperature,
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increase of ocean editification
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which is a negative impact on marine life.
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Decrease of the oxygen always a very negative
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impact on marine life, and because the ocean
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is becoming warmer, sea level will be rising.
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So there is no other solution
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than decreasing our greenhouse gases emissions
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and turning towards other energy sources.